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Abstract:

The X-ray background (XRB) and the intergalactic medium (IGM) are two of the most interesting topics in X-ray cosmology. Historically, studies of either subject provided important information on the other. For more than 30 years, the problem of the origin of the XRB has remained unsolved, despite large advances in X-ray instrumentation. Along the way, important constraints have been placed on the state of the intergalactic medium. Based on theoretical modelling and observational constraints, I have investigated both the origin of the X-ray background and the physical state of the intergalactic medium in this thesis. The first Chapter gives an outline of the observed properties of the XRB and the manner by which X-ray studies constrain the IGM. Chapter 2 reviews previous observations of the XRB and our current understanding of the spectrum, isotropy, and the resolved component of the XRB. An analysis of ROSAT data to study the spatial structure of the XRB is presented in Chapter 3. The ROSAT PSPC data include one deep survey field and six colinear medium survey fields, from which I have obtained limits on excess fluctuations and the autocorrelation function (ACF) of the XRB in the soft X-ray energy band from 1 arcmin to 5 deg. The 2σ upper limits of these results show no strong signal of anisotropy and inhomogeneity. Incorporating the observed ACF into detailed models of evolving discrete sources, I find that the correlation length of the major source dominating the residual XRB has to be < 4h50-1 Mpc, which is less than that of normal galaxies. Spectral fitting of both resolved and unresolved components of the XRB was carried out and is shown in Chapter 4. Data from the QSF3 field observed with the ASCA SIS and the ROSAT PSPC were used for this task. The diffuse XRB has a flat spectrum, which is well fit by a single power-law extending from 0.1 to 7 keV, with a photon index ranging from 1.4 to 1.5. On the contrary, the spectrum of the resolved component appears steeper. Such a steep-spectrum component can contribute in total less than 30 per cent to the residual XRB below 3 keV and only a few per cent above 3 keV. This suggests the entire XRB is composed of two spectrally distinct populations, of which the one with a steeper spectrum has been resolved in the soft band and most of which are AGN.